In this paper, the effect of internal curing with pre-soaked lightweight aggregate (PSLWA) on shrinkage and interior relative humidity of four series concretes with compressive strength at 28 days around 30MPa, 60MP, 90MPa and 100MPa is investigated. The shrinkage induced cracking performance of concretes is evaluated with concrete-steel composite ring tests. The results show that the development of the internal relative humidity of concrete since casting exhibits first a vapor saturated stage (RH=100%, stage I), followed by gradually reducing stage (RH<100%, stage II). Under sealing, internal relative humidity at the center of the specimen for a given age is obviously decreased with increase of concrete strength. Under drying, similar internal relative humidity value is observed at 28 days for all concretes without internal curing. As PSLWA was added, the reduction rate on interior humidity in stage II is significantly decreased. But the efficiency on internal humidity rising is greatly influenced by concrete strength. The autogenous shrinkage is in-creased with increase of concrete strength. The drying shrinkage is decreased with increase of concrete strength. The total shrinkage of concrete is increased with increase of concrete strength. As PSLWA was added, the shrinkage reduction in both autogenous and drying shrinkages is obviously in high strength concretes, such as 90MPa and 100MPa concretes. The shrinkage reduction of internal curing on relatively low strength concrete, such 30MPa and 60MPa concrete is not obvious. Internal curing with PSLWA can greatly improve the shrinkage induced cracking performance. All concrete rings without internal curing are cracked under drying. The compressive strain in the steel ring at the concrete ring cracking is about 75 to 101μm/m. By contrast, the stable compressive strain at the inner steel ring for the concretes with internal curing becomes 10 to 40μm/m and no visible cracks were found on the specimens.
In line with the strategic energy goals and sustainable development, considerable investment has been made on nuclear energy, leading to construction of a growing number of nuclear power plants (NPPs) across China. Concrete is the main structural materials applied in many key elements of a typical NPP including reactors. Among other stringent require-ments, resistance to thermal loads both at early ages and during operating life is crucial for the concrete used in nuclear facilities. This paper provides an overview of high temperature resistance of concrete structures in Chinese NPP, including information on construction and operation. Early-age thermal cracking of mass concrete structures due to dif-ferential thermal stresses induced by hydration heat has been highlighted in literature as a major issue. The common approaches to tackling this issue are based on limiting the maximum temperature in the concrete and temperature difference between hotter interior and cooler exterior of the concrete. These approaches include the optimum mix ratio method and the reasonable design of the construction technology. During operation, the temperature control of containment under steady or accidental case is an important prerequisite to ensure the safety of NPP, especially after a loss of coolant accident (LOCA). In addition, various coatings used for thermal insulation in China’s NPP are introduced and compared. Based on the existing literature, further studies involving longtime monitoring of temperature, strain and displacement are found necessary to obtain a better understanding of thermal resistance, long-term performance and safety of concrete containment used in NPP.
In this paper, we present a study about the assessment of structural feature and ionic diffusivity of interfacial transition zone (ITZ) in blended cementitious composites, which are made of Portland cement, blast furnace slag and limestone filler. In particular, three mortar series are examined with respect to varying replacement levels of supplementary cementitious materials (SCMs), curing age and water-to-binder (w/b) ratio. Based on the techniques of experimental measurement and numerical simulation, i.e., the backscattered electron (BSE) image analysis and the HYMOSTRUC model, the structural features of ITZ in designed mortars are elaborated in a quantitative manner. Thereafter, a lattice Boltzmann method based computer modeling of ionic diffusion in the ITZ is performed to predict the ionic diffusivity. Results indicate that the ITZ differs much from bulk paste, which is of significant importance in blended cementitious composites. The structural feature and the ionic diffusivity of ITZ are prominently affected by various casting factors, such as replacement levels of SCMs, curing age and w/b ratio.
Among the necessary data in the description of the transport of water, the retention curve is a vital piece of information whose characterization can turn out to be long and fastidious. Within this framework, sorption balance is being put to use more and more often in laboratories. These experimental devices enable a faster onset for water balance using small amounts of substances previously reduced to powder. This approach proves to be particularly adapted to homogenous materials (cement pastes generally) but not to concrete because of the presence of aggregate. This article, first of all, clarifies in a summarized manner the working of sorption balance based on the results obtained with cement pastes. Secondly, a simplified method is proposed to test concrete.
A review of the current state of knowledge on the effects of radiation on concrete in nuclear power production applications is presented. Emphasis is placed on the effects of radiation damage, as reflected by changes in engineering properties of concrete, in the evaluation of the long-term operation and for plant life or aging management of nuclear power plants (NPPs) in Japan, Spain, and the United States. National issues and concerns are described for Japan and the United States followed by a discussion of the fundamental understanding of the effects of radiation on concrete. Specifically, the effects of temperature, moisture content, and irradiation on ordinary Portland cement paste and the role of temperature and neutron energy spectra on radiation-induced volumetric expansion (RIVE) of aggregate-forming minerals are described. This is followed by a discussion of the bounding conditions for extended operation; the significance of accelerated irradiation conditions; the role of temperature and creep; and how these issues are being incorporated into numerical and meso-scale models. From these insights on radiation damage, analyses of these effects on concrete structures are reviewed, and the current status of work in Japan and the United States is described. Also discussed is the recent formation of a new international scientific and technical organization, the International Committee on Irradiated Concrete, to provide a forum for timely information exchanges among organizations pursuing the identification, quantification, and modeling of the effects of radiation on concrete in commercial nuclear applications. The paper concludes with a discussion of research gaps, including (1) interpreting test-reactor data, (2) evaluating service-irradiated concrete for aging management and to inform radiation damage models with the Zorita NPP (Spain) serving as the first comprehensive test case, (3) irradiated-assisted alkali-silica reactions, and (4) RIVE under constrained conditions.
In the Hokuriku district in Japan, large numbers of concrete structures have been suffering from damage caused by alkali silica reaction (ASR). Some huge volcanoes are located within this district, and the headwaters of main rivers prompt the outflow and spreading of volcanic rocks such as andesite, rhyolite and tuff stones, which are the main volcanic reactive stones causing the serious damage of ASR in the entire area. To solve this problem effective countermeasures such as repair and strengthening methods should be established.
One intake tower in this area had deteriorated due to ASR, and deformation had occurred as a consequence of ASR ex-pansion of the concrete. Countermeasures were carefully considered by academic experts, and post-tensioned tendons were inserted into the intake tower concrete (vertically oriented) so that the power station can continue to operate safely in the future. This is thought to be the first challenge of its type anywhere in the world for which the deformation of a real structure caused by ASR expansion must be controlled.
In this paper, the results obtained from laboratory tests using reactive aggregate and the overview of the investigation and the method of the reinforcement including the effect of the countermeasures will be discussed.
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